Among families of antimicrobial proteins associated with plant defense are the pathogenesis related proteins of family 5 (PR-5) that are structurally related to the sweet tasting protein thaumatin (Veronese et al., 2003). PR-5 proteins are distinguished by having (a) a three domain structure consisting of a β barrel (Domain I) that forms the compact core of the molecule (this structure is commonly found in lectins and is hereafter referred to as a “lectin-like β barrel”); a domain (Domain II) that extends from Domain I and consists of several loops stabilized by four disulfide bonds; and Domain III that also extends from domain I and consists of a small loop stabilized by two disulfide bonds, (b) an alanine commonly located at the cleavage site of the N-terminal leader sequence, (c) up to 16 cysteine residues that have a conserved spatial distribution throughout the protein and linked by disulfide bridges (Min, et al., 2004), and (d) a cleft formed by Domains I & II that could be associated with biological activity.
Osmotin is an antifungal tobacco PR-5 protein. It induces programmed cell death in S. cerevisiae by signaling suppression of cellular stress responses via RAS2/cAMP (Narasimhan et al., 2001). Most of the PR-5 proteins, including osmotin, have specific wide-spectrum antifungal activities, suggesting that target recognition may be determined by their interaction with pathogen cell surface components. In the case of osmotin, specific fungal cell wall components enhance or suppress osmotin antifungal activity (Narasimhan et al., 2003; Veronese et al., 2003). In S. cerevisiae, the PIR family of cell wall glycoproteins are osmotin-resistance determinants (Yun et al., 1997). Phosphomannans of yeast cell wall glycoproteins have been reported to increase osmotin toxicity, probably by serving as docking structures for osmotin, thereby increasing its local concentration and diffusion across the cell wall (Ibeas et al., 2000). Genetic analyses have revealed that SSD1, a protein that affects cell wall morphogenesis and deposition of PIR proteins, is a determinant of resistance to osmotin (Ibeas et al., 2001). Unidentified changes in the yeast cell wall that enhance toxicity are induced by osmotin via activation of a mitogen-activated protein kinase cascade (Yun et al., 1998). Specific interactions at the plasma membrane also appear to be required for osmotin antifungal activity because yeast spheroplasts that are susceptible to tobacco osmotin can be resistant to PR-5 proteins from other plant species (Yun et al., 1997). Although the best studied role of PR-5 proteins is their antifungal activity, a signaling or recognition role has been suggested. Veronese et al., 2003
Adiponectin (also called 30-kDa adipocyte complement-related protein-Acrp30) is an antidiabetic and antiatherosclerotic protein hormone in mammals that conditions sensing of energy status, fatty acid oxidation and glucose transport upon interaction with adiponectin receptors. Known adiponectin receptors include the human AdipoR1 and AdipoR2 (Diez and Iglesias, 2003; Yamauchi et al., 2003a,b), and the pig adiponectin receptor genebank NM—001007193) (Ding et al. 2004). Additional adiponectin receptor sequences can be found in GeneBank. Serum adiponectin levels are decreased under conditions of obesity, insulin resistance, and type II diabetes (Yamauchi, et al., 2003a), while administration of adiponectin lowers serum glucose levels and ameliorates insulin resistance in mice (Yamauchi, et al. 2003a). The mammalian adiponectin receptors AdipoR1 and AdipoR2 are predicted to have seven transmembrane domains, a characteristic feature of G protein-coupled receptors (GPCRs).